What type of cells are Antigen-Presenting Cells (APCs)?

Antigen-Presenting Cells, or APCs, play a crucial role in the immune response by presenting antigens to T cells. These cells include several types of immune cells, most notably dendritic cells, macrophages, and B cells, all of which express MHC Class II molecules on their surface.

The MHC Class II markers are essential for the function of APCs, as they allow these cells to present processed foreign antigens to CD4+ T helper cells. This interaction is fundamental for the activation and proliferation of T cells, leading to a robust immune response. APCs can also present antigens to CD8+ T cytotoxic cells, though this typically involves MHC Class I molecules.

While it's true that all APCs are immune cells, the defining characteristic that makes them antigen-presenting is their expression of MHC Class II markers. This specificity sets them apart from other types of cells, such as leukocytes that are not APCs. Consequently, the defining characteristic of the correct answer identifies the essential role and identity of these cells within the immune system.